1. Field of the Invention
This invention relates to thermal printing heads and, more particularly, to substrate-supported thermal printing elements.
2. Description of Prior Art
Referring to FIG. 1, there is shown a schematic representation, in cross-section, of a typical thermal printing element 10 which is known in the art. The element 10 comprises a substrate 11 typically of glass or other rigid, thermally insulating material; a resistor layer 12, typically of cermet, supported by the substrate; and a metallic layer 13 of conductors supported by the resistor layer 12. The element 10 can be formed by sputtering a layer 12 of cermet on a glass substrate 11, sputtering a layer 13 of copper on the cermet layer 12, etching a composite resistor-conductor pattern through the conductor and cermet layers, and then etching the conductor layer 13 to remove the copper from areas of the cermet layer 12. This leaves exposed, recessed cermet resistor segments 14 which form the operating portion of the thermal print element 10. As will be appreciated by those skilled in the art, the conductor layer 13 substantially eliminates I.sup.2 R heating of those portions of the resistor layer 12 disposed thereunder, but supplies current through the exposed resistor segments 14 to heat the exposed resistors. FIG. 1 is merely illustrative of a typical cross-section and does not show the many possible configurations for the resistor segments 14 nor the electrical connections which are used to complete current paths through the resistor segments.
The conductors of layer 13 typically have a thickness of about 15,000 A. As a result, the resistors 14 are recessed 15,000 A relative to the conductors and are separated from the recording medium, such as thermal printing paper, (not shown) by at least the same distance. Consequently, the printing paper must be brought into contact with the resistors 14, typically by rollers which back the paper. The small openings between the conductors makes this difficult.
In addition, the conductors formed in layer 13 must be aligned with and connected to external electrical leads (not shown) which supply the current to the resistors 14 via the conductors. Of course, the alignment and the use of clamps or other means to mount the external leads to the conductors adds to the cost of manufacturing. In addition, the clamps or other holding means may overlap onto the printing paper, requiring additional movement of the paper in order to read the printing.
As may be appreciated, it is highly desirable to have a resistor structure which will eliminate problems such as the above-described problems of printing, aligning, mounting and reading.